A typo in utm_source doesn’t lose any data. Every session still gets recorded, still gets counted, still shows up somewhere in GA4. What it loses is the ability to see those sessions as one thing.
Three people build links for the same campaign over three days: instagram, Instagram, isntagram, and GA4 has no reason to treat them as related. Ten thousand real sessions become three separate rows of a few thousand each, every one of them looking like a mediocre campaign instead of the strong one it actually was. Whoever’s deciding whether to run it again next quarter is reading the wrong number, without knowing it’s wrong.
Finding this out costs real time, just never on a schedule anyone plans for. It shows up as a nagging sense that a report doesn’t quite add up, then twenty minutes cross-referencing source strings in Explore, then the moment it clicks that three unrelated-looking rows were the same campaign all along. None of that lands on a task list. It’s just quietly there, once a quarter or so, for every team hand-building links at any real scale.